A method for reading magnetically stored information, especially information stored in a binary system, from a magneto-optical or optoelectronic storage medium can use a light beam directed upon the storage medium and either reflected thereby or transmitted therethrough.
A light beam coming from the storage medium and either a reflected light beam or a transmitted light beam (hereinafter the outgoing light beam) is received by an optoelectric detector, especially a photodiode which transduces the outgoing light beam into an electrical signal V.sub.M.
The acceptable range for V.sub.M is a voltage larger than 0 but smaller than the working voltage of the electronic circuit (typically 10 V, as determined by the amplifiers). In the case that the voltage that can be handled by the electronic components is larger than 10 V, then the upper limit for V.sub.M is given by this larger voltage.
This electrical signal can be processed by an electronic circuit which can include a preamplifier, an amplifier and a data processing system so that the magnetically stored information can be converted into electronic data and processed in the usual manner with binary or like coded stored information.
An apparatus for this purpose generally comprises a light source from which the incident beam derives, a magneto-optical or optoelectronic storage medium, the light/electric detector or transducer and an electronic circuit supplied by the transducer and outputting the electronic pulses representing the stored information. The electronic circuit can include a preamplifier, a compensator for modifying the preamplified signal, a main amplifier for amplifying the modified signal, and the data processing system transforming the amplified signal into corresponding electronic pulses representing the stored data.
The storage medium is usually a layer system with appropriate magnetic characteristics and which can be used as magnetic storage in the digital information processing art, the magnetic layer being applied to a nonmagnetic carrier. Typical layers can have thicknesses of the order of 1.times.10.sup.-6 m and even thinner layers can be used with thicknesses as low as 1.times.10.sup.-10 m. However, there are no limits for the maximum thickness which can be detected Reference may be had in this regard to J. G. Gay, R. Richter, J. Appl. Phys. 61, 3362 (1987) and F. J. A. den Broeder et al., Phys. Rev. Lett 60, 2769 (1988). The storage process in such a layer storage medium requires that the storage medium be transformable into two different magnetic states which, for convenience below will be referred to as the +1 and -1 states.
The switchover between these two magnetic states is generally due to a small externally applied magnetic field H. The so-called magnetic memory of the storage medium is reflected in the fact that a particular applied magnetic state will remain or be retained even when the external magnetic field H is removed.
The magnetic state of the stored medium can be described as a remanant magnetization M and is, in effect, a retained electromagnetic field of a particular orientation The states +1 and -1 represent, therefore, the possible saturation values of the magnetization of the storage medium and can represent polarization in two opposite directions.
The property of the magnetization M of a storage medium as a function of the externally applied magnetic field H, is described by the so-called hysteresis curve. The knowledge of the form of the hysteresis curve enables a determination of the so-called coercivity field H.sub.C at which the magnetization can be switched over from the state +1 to the state -1 and vice versa and the determination of the so-called remanence, i.e. the residual magnetization for an applied magnetic field H=O.
For maximum storage density of the medium, the value H.sub.C should be optimized and, as a consequence, the dimensions of the domains which can assume the magnetic states +1 or -1 independently from one another should be minimized. The knowledge of the hysteresis curve, therefore, allows a quality control of the fabrication of the storage medium.
It is also known in the art to provide a process for reading magnetically stored information based upon the so-called magneto-optical Kerr effect.
In accordance with this phenomenon, the intensity and polarization of the light reflected or transmitted from a specimen are affected by the magnetic state of the specimen. This is based upon the physical effect on an elementary optical process, for example, reflection because of the influence of the magnetization M of the storage medium on the Kerr effect thereof.
State of the art apparatus for utilizing this process in reading magnetically stored information can be very complicated and bulky. The process itself is rather complex.
German patent document 29 53 304 (col. 4, lines 17-19) describes a device for reading magnetically stored information in which the light beam outputted by the light source is elliptically polarized. German patent document 21 2 1 510 (col. 1, lines 10-20) describes an apparatus for reading a binary information storage utilizing the Kerr effect in which a magnetic mirror is arranged in the path of the light beam from the storage layer and comprises a ferromagnetic metal. In addition, an arrangement is proposed in which the ferromagnetic metal of the mirror can be brought to magnetic saturation in at least one of the magnetization directions to correspond to a value of a binary digit in the storage layer.
These state of the art processes are not only very complicated but they also require complex equipment which can be bulky. In addition they are frequently characterized by the drawback that they do not have a sufficient sensitivity with respect to determination of the hysteresis curve of the storage medium especially when very thin layers in the range of 10.sup.-10 m, or so-called monolayers, are involved.